Abstract

A nonisotopic, immunoelectrophoretic technique was used to analyze the characteristics of opioid-evoked activation of Gi2/Gx/z transducer proteins of mouse periaqueductal gray matter membranes. In the presence of picomolar concentrations of guanosine 5′-O-(3-thiotriphosphate), the opioid agonists promoted concentration-dependent increases of immunoreactivity associated with free Gi2α and Gx/zα subunits. [d-Ala2,N-MePhe4,Gly-ol5]enkephalin and morphine (preferential agonists at mu opioid receptors) and β-endorphin-(1–31) (an agonist atmu/delta opioid receptors) activated Gx/zproteins. In contrast, the agonists of delta opioid receptors, [d-Ala2]deltorphin II and [d-Pen2,5]enkephalin, displayed little or no activity on this pertussis toxin resistant regulatory protein. Although exhibiting diverse efficacy, all the opioids studied activated Gi2 transducer proteins. [d-Ala2,N-MePhe4,Gly-ol5]enkephalin and [d-Ala2]deltorphin II were more potent at Gi2α subunits than at Gx/zα subunits. The opioid antagonist naloxone displayed a competitive profile in reducing the activation of G proteins promoted by morphine. Moreover, [d-Pen2,5]enkephalin antagonized the releasing effect exerted by [d-Ala2]deltorphin II on Gi2α and Gx/zα subunits.N,N-diallyl-Tyr-Aib-Aib-Phe-Leu (ICI-174864) reduced the Gα-related immunosignals promoted by agonists of delta opioid receptors. Therefore, it is suggested that opioids exhibit marked differences in efficacy and/or potency in the activation of Gi2 and Gx/ztransducer proteins in mouse periaqueductal gray matter.

Opioid receptors belong to the superfamily of receptors that regulates GTP-binding proteins (G proteins). The ability of a receptor to regulate different classes of G proteins causing the productive activation of multiple G protein signaling pathways has been convincingly documented (Kenakin, 1995). Much work has been performed to characterize transduction regulated by opioid receptors, not only in in vitro but also in in vivo systems, e.g., the production of antinociception. The i.c.v. injection of pertussis toxin into rodents demonstrated the regulation of Gi/Go families bymu/delta opioid receptors in the mediation of analgesia (Parenti et al., 1986; Sánchez-Blázquez and Garzón, 1988, 1991; Lutfy et al., 1991). To gain information about the transduction system implicated in analgesia mediated by mu and delta opioid receptors, the authors’ group pioneered in vivo studies with antibodies to Gα subunits. Possible neural processes facilitating the access of IgGs to G proteins have been discussed (Garzón, 1995). Supraspinal antinociception produced by agonists of deltareceptors was reduced in mice receiving antibodies to Gi2α and Gi3α subunits by i.c.v. injection (Sánchez-Blázquez and Garzón, 1993;Sánchez-Blázquez et al., 1993). However, the analgesic profile of mu opioid receptor agonists appeared greatly diminished after injecting antibodies to pertussis toxin-sensitive Gi2α and pertussis toxin-insensitive Gx/zα subunits (Sánchez-Blázquez et al., 1993, 1995). The assignment of G proteins to muand delta opioid receptors has been substantiated by an alternative approach; the in vivo administration of antisense oligodeoxynucleotides complementary to portions of mRNAs expressing different subtypes of Gα subunits (Raffa el al., 1994; Rossi et al., 1995;Sánchez-Blázquez et al., 1995; Tang et al., 1995; Standifer et al., 1996).

Of potential interest was the finding that after reducing the availability of a single class of Gα subunits, agonist-antagonist properties of opioids were revealed (Garzón et al., 1994; Sánchez-Blázquez and Garzón, 1988). Because the various classes of Gα subunits exhibit structural differences in their C-terminal peptide sequences, it is feasible that agonist-bound receptors should display diverse efficacy in the activation of the subtypes of G proteins (Roerig et al., 1992; Chabre et al., 1994; Law et al., 1994; Lui et al., 1994; Offermanns et al., 1994). To obtain further insight into receptor-mediated effects, it is important to determine the classes of G-proteins regulated and their order of activation. The efficacy of activation of all, or some, of the G proteins regulated by a given receptor should shed some light on the agonist-antagonist properties of the ligands.

A number of laboratories have described a regulated association of G protein subunits with cytoskeletal proteins (see references in Neubig, 1994). In immunocytochemical studies, G proteins are often viewed as clusters distributed along the plasma membrane (Lewis et al., 1991). The finding that Triton X-100 solubilization of cell membranes provides insoluble complexes enriched in different types of proteins, receptors and GTP-binding proteins included, suggests that this constrained distribution corresponds to that of noncoated pits or caveolae-sites enriched in signal transducing elements (Neubig, 1994;Sargiacomo et al., 1993). In agreement with these findings, heterologous aggregates of proteins containing Gα-like immunoreactivity have been isolated after mild solubilization of cell membranes (Coulter and Rodbell, 1992; Jahangeer and Rodbell, 1993). Further, disaggregation was achieved using guanine nucleotides or extensive solubilization of the membranes (Nakamura and Rodbell, 1990). In the present investigation the characteristics of the GTP-dependent opioid-induced activation of different G proteins in the cell membrane were investigated using the rocket immunoelectrophoretic technique described by Laurell (1966).

An investigation was made of the activation of Gi2α and Gx/zα subunits by agonists binding mu and/ordelta opioid receptors in membranes from mouse PAG. This neural structure plays a major role in mediating the supraspinal analgesic effect of opioids when given by i.c.v. injection (Yakshet al., 1976; Jensen and Yaksh, 1986). The results of this study indicate that mu opioid receptors couple with the Gi2 and Gx/z types, whereas deltaopioid receptors prefer Gi2 over the Gx/z type. The observation that opioids displayed dissimilar “efficacy” atmu/delta opioid receptors in the activation of these Gi2/Gx/z transducer proteins is of potential interest.

Methods

Membrane Preparation

Experimental tissue was provided by albino male mice CD-1 (Charles River, Barcelona) weighing 22 to 25 g. P2fractions from PAG were prepared as previously described bySánchez-Blázquez and Garzón (1989). Membranes were diluted in Tris.HCl buffer, pH 7.7, to a final protein concentration of 2 μg μl−1.

Incubation of P2 Membranes with GTPγS and Opioids

Concentrations of GTPγS ranging from 1 fM to 300 μM were incubated with 40 to 80 μg protein from mouse PAG in 30 to 50 μl (25 mM Tris.HCl, pH 7.7) for 60 min at 25°C. The nucleotides GDP, GDPβS and ATPγS were used at 100 μM. The opioid agonists, morphine, β-endorphin-(1–31), DAMGO, DPDPE, [d-Ala2]deltorphin II and the opioid antagonists, naloxone and ICI-174864, were incubated in this buffer supplemented with 1 pM GTPγS. The interaction between opioids was allowed by adding a fixed concentration of the first ligand 10 min before a range of concentrations of the second ligand, followed by incubation for 50 min. The samples, membranes plus incubation medium, were SDS-solubilized (boiled for 3 min at 100°C) and the final ratio being 2 μg SDS μg−1 membrane protein. For immunoelectrophoresis, 2 to 4 μg protein were transferred to each sample well in volumes of 4 to 6 μl.

In a series of assays, the membranes were incubated for 60 min at 25°C with 1 μM morphine in the presence of 100 pM guanosine 5′[γ-thio]triphosphate [35S] (New England Nuclear, Boston, MA, NEG-030H, 1244 Ci/mmol). Samples were centrifuged at 20,000 × g for 20 min. The precipitate and the supernatant were SDS-solubilized and immunoelectrophoresis was performed in agarose plates loaded with anti Gi2α antiserum (fig. 1). After this the dried plates were exposed to X-OMAT film (Kodak). Films were developed in Kodak LX-24 developer and Kodak AL-4 fixer.

Effect of GTP/GTPγS on G protein α-like immunoreactivity from mouse PAG membranes. A, Mouse PAG membranes were incubated with 100 μM GTPγS, solubilized with 2 or 8 μg SDS μg−1 protein and immunoelectrophoresed in agarose plates containing anti-Gi2α serum. Control samples were incubated without GTPγS. B, Time-course for the GTP/GTPγS-induced increases of Gα-related immunosignals. The guanine nucleotides were incubated at 10 μM with the PAG membranes. The samples were SDS-solubilized (2 μg μg−1 protein) at different intervals and aliquots of 4 μg were electrophoresed in agarose gels containing anti-Gi2α serum. The immunoreactivity is expressed relative to that found in the absence of GTP/GTPγS incubation (percentage of increment). C and D, A range of GTPγS concentrations were incubated with the membranes for 60 min. After solubilizing the samples (2 μg SDS μg−1 protein), 4 to 6 μg membrane protein were studied for immunoreactivity in agarose gels containing anti-Gi2α (C) or anti-Gx/zα serum (D) Values are the mean ± S.E.M. from three to four independent determinations. Bottom left: Actual appearance of the immunoprecipitates. From 1 to 5 μg SDS-solubilized (2 μg μg−1 protein) PAG membranes were electrophoresed in an Gx/zα serum-agarose gel. Incubated in the absence (left) or presence (right) of 100 μM GTPγS. Bottom right, Autoradiographic image of agarose plates loaded with antibodies to Gi2α subunits. Samples were incubated with (wells 1 and 2) or without 1 μm morphine (well 3) in the presence of 100 pM guanosine 5′[γ-thio]triphosphate [35S]. Radioactivity associated with Gi2α-like immunoreactivity detected in: 1, the supernatant; 2, the fraction of membranes; 3, supernatant plus membranes.

Immunoelectrophoresis.

The method used was that described byLaurell (1966) with minor modifications (Dunbar, 1987). This nonisotopic procedure is based on the differential migration of the antigen and immunoglobulins in an appropriate buffer system. The electrophoretic migration of IgGs is very low at pH 8.6 whereas the Gα subunits with isoelectric point values in the range 5.4 to 6.1 (Goldsmith et al., 1988; Spicher et al., 1992) migrate rapidly in the electric field. As the antigen moves to the anode the content of IgGs inside the migrating track becomes reduced and the insoluble antigen-IgG complexes construct the sides of the rocket-shaped immunoprecipitate. When no antigen is available the immunoprecipitate ends in a sharp peak. The Gα subunits in complexes with other proteins or included in the aggregates, are bound very poorly by specific antibodies. The accessibility of the IgGs is enhanced after promoting the release of Gα subunits by either aggressive solubilizations, high concentrations of guanine nucleotides or agonist-liganded receptors (Nakamura and Rodbell, 1990). The final effect would be as if the amount of antigen had increased, thus producing greater immunosignals; the stoichiometric ratio Gα/IgG was reduced.

Agarose gel preparation.

A solution of 1.5% (w/v) agarose (FMC no. 50002, LE) was prepared in electrode buffer: 20 mM Tris.HCl, 25 mM sodium barbital, 5 mM barbital, 0.85 mM calcium lactate, 0.03% (w/v) sodium azide (pH 8.6). Agarose was dissolved by constant stirring in the heated buffer (90°C, microwave oven). When cooled to 45°C, immune serum was added at 1:100 dilution. Once the solution became clear and thickened, 12 ml of the molten agarose were poured over the whole surface of leveled 85 × 100 mm plastic plates (GelBond FMC no. 53734). The casted gels were first cooled to room temperature and then placed in a humidity chamber at 4°C for at least 2 hr before use. Samples were applied to the gel in 2.5-mm round wells of approximately 7 μl capacity cut with a gel punch connected to a vacuum source. The gel minus the well area was covered with a plastic film and immunoelectrophoresis performed in a humid atmosphere on a cooling plate (LKB 2117 Multiphor II Electrophoresis System) connected to a thermostatic circulator bath (Haake G/3D) set at 10°C. Samples containing the antigen were loaded into the punched holes on the cathode side. A constant current of 7 mA per agarose gel plate was applied for 18 hr (Isco Power Supply 452). Antigen molecules migrated toward the anode and the insoluble immunocomplexes formed the rocket-shaped precipitate. A relationship between the height of the “rocket” and the amount of antigen applied was obtained (Laurell, 1966; Dunbar, 1987).

Staining and quantification.

The agarose gel was separated from the plastic support. To remove the unreacted proteins the gel was placed between sheets of chromatography paper (Whatman 3 mm CHR) soaked in 100 mM NaCl and pressed (1 kg) for 2 × 30 min. The gel was then immersed and gently agitated in this saline solution for 20 min. After a second pressing for 30 min, the gel was dried under a warm air stream before staining. Immunocomplexes, containing the Gα-like immunoreactivity plus the specific IgGs, were stained with Coomassie Brilliant Blue R-250 (BioRad). Silver staining of the “rockets” was also performed with reagents suitable for agarose gels (Silver Stain Plus, BioRad), although increases in the signal were accompanied by higher backgrounds. The length of the immunoprecipitates—distance from the center of the well to the peak of the rocket—was assessed by optical densitometry (Isco Gel Scanner 1312/UA-5).

Statistics

The data are expressed as the mean ± S.E.M. of at least three independent experiments. Statistical significance was assessed using the Student’s paired t test. P < .05 were considered to be significant. Calculations were performed using the SigmaStat computer program (Jandel Scientific Software, Erkrath, Germany).

Results

Detection of Gα subunits by immunoelectrophoresis.

Mouse PAG membranes solubilized with SDS in the absence of thiol-reducing agents were loaded into agarose gel plates containing anti Gi2α or anti Gx/zα serum at 1:100 dilution (fig.2A). The length of the immunoprecipitates “rockets” increased linearly with the amount of protein undergoing electrophoresis. The curves correlating protein concentrations to immunosignals did not pass through the origin and showed a threshold for immunodetection. The concentration of the negatively charged detergent SDS in the solubilizing buffer influenced the size of the immunosignals. Increases in the ratio SDS/protein extended the thresholds and parallel curves could be observed (fig. 2A). Samples solubilized with ratios SDS/protein less than 0.5 hardly entered the agarose gel and the stained proteins were detected in the sample wells (not shown). For the purpose of the study, a constant ratio of 2 μg SDS μg−1 protein was selected. Immunodetection was achieved at low micrograms of membrane protein (1–6 μg) indicating that Gα subunits are detected in the nanogram range. The method is highly reproducible, the S.E.M. of the computed immunoprecipitates is less than ± 2% and the accuracy of the measurements increases with the length of the rockets (Laurell, 1966). In assays where differences are expressed as a percentage of the control, the threshold “blank” was subtracted from the length of the rockets.

Proteins other than the Gi2α/Gx/zα subunits, provide no immunoprecipitates, e.g., bovine serum albumin. Moreover, in agarose gels containing preimmune sera, or immune sera boiled at 100°C for 10 min, the immunogens did not produce the rocket-shaped precipitates (data not shown).

Incubation of samples with GTP/GTPγS produced an enhancement of the Gα-related immunosignals (fig.2). The increases in immunoreactivity observed after raising the amount of SDS in the solubilizing buffer masked the effect of 100 μM GTPγS (fig. 2A, bottom left). Samples incubated with GTPγS and solubilized with 2 μg SDS μg−1 protein gave steeper curves than those produced by the corresponding controls. However, the threshold was practically maintained (more antigen being available for the IgGs) (fig. 2A). The time-course for the enhancing effect of GTP and GTPγS on Gi2α-like immunoreactivity is shown (fig.2B). A concentration of 10 μM GTP/GTPγS, was incubated with mouse PAG membranes at several intervals before stopping the reaction (SDS-solubilization). The GTPγS raised the Gi2α-related immunoreactivity over the basal values (obtained in the absence of the nucleotide). The pattern showed a fast rise during the first 15 min followed by a steady plateau that extended beyond 120 min a result in agreement with that reported by Milligan and Unson (1989) who used membranes of rat glioma C6 BU1 cells and Gpp(NH)p. A concentration of 10 μM GTPγS promoted a steady increase of Gx/zα-related immunoreactivity after 40 to 50 min of incubation (not shown). An interval of 60 min was therefore selected to carry out further assays with GTPγS. The effect of GTP showed a biphasic pattern on Gα-like immunoreactivity: an initial rise lasting up to 30 min followed by a pronounced decrease that practically returned to the basal levels. The metabolism suffered by GTP was responsible for the reversible activation of the Gα subunits. At the end of the incubation period the fraction of activated Gα subunits depends on the concentration of GTP that remains. Considerable loss of sensitivity and experimental variation follows.

Incubation of mouse PAG membranes with GTPγS brought about concentration-dependent increases of the immunoprecipitates. A concentration of 6.5 ± 0.6 μM GTPγS produced a 50% increase of the basal Gi2α-like immunoreactivity of PAG membranes (fig. 2C). For Gx/zα-related immunoreactivity the computed ED50 was 800 ± 12 nM GTPγS (fig.3D). This simple and sensitive method permitted immunodetection of GTP-activated Gα subunits. The potency (ED50) of GTPγS to increase Gi2α-like immunoreactivity compares satisfactorily with that reported byMilligan and Unson (1989) using Gpp(NH)p. These authors report that 0.1 to 1 mM Gpp(NH)p produced the greatest activation of Gα subunits. In our study 0.1 to 0.3 mM GTPγS achieved this effect.

Specificity of the Gα-related immunosignals detected in immunoelectrophoresis. A, PAG membranes were incubated in the absence, and also in the presence of 100 μM GDP/GDPβS for 45 min at 25°C. GDP was removed before incubation for an additional 60 min with 100 μM GTPγS. Samples were also ADP-ribosylated with pertussis and cholera toxin as described in “Methods” before being incubated with or without 100 μM GTPγS. The solubilized samples (2 μg SDS μg−1 protein) were electrophoresed in agarose gels containing anti-Gi2α serum. * Significantly different to the corresponding group incubated in the absence of GTPγS (open column). †Significantly different to the control group when incubated in the absence of GTPγS (open column), P < .05, Student’s t test. B, Influence of 5% mercaptoethanol on GTPγS-mediated enhancement of Gi2α-like immunoreactivity. The membranes were incubated in the absence and presence of 100/300 μM GTPγS and solubilized with and without 5% mercaptoethanol in the solubilization buffer (2 μg SDS μg−1 protein). * Significantly different to the group solubilized without mercaptoethanol (open column). †, Significantly different to the Control group incubated in the absence of GTPγS and solubilized without mercaptoethanol (open column), P < .05, Student’s t test. The empty boxes drawn for panels A and B indicate the threshold for immunodetection (no rocket is formed below this migration distance). C, Influence of GTPγS and mercaptoethanol on deltareceptor-related immunoreactivity. The membranes were incubated in the presence and absence of 100 μM GTPγS and SDS-solubilized with and without 5% mercaptoethanol before electrophoresing the samples in agarose gels containing anti-delta opioid receptor serum.

After incubating membranes with 1 μm morphine and 100 pM guanosine 5′[γ-thio]triphosphate [35S] the labeled nucleotide was detected bound to Gi2α-like immunoreactivity in both fractions, membranes and supernatant. This was not observed when incubation was conducted without the opioid (fig. 2, bottom right).

Opioid-induced GTPγS-dependent activation of G transducer proteins.

The opioid agonists, morphine, β-endorphin-(1–31), DAMGO, DPDPE, [d-Ala2]deltorphin II, or the antagonists, naloxone and ICI-174864, when incubated in 25 mM Tris.HCl, pH. 7.5, in the absence of GTPγS, did not modify Gα-related immunosignals. Concentrations of GTPγS in the range 1 fM to 1 μM were subeffective or produced a limited increase of the Gi2α-like immunoreactivity from PAG membranes (figs. 2C and 4A). Samples preincubated with combinations of 1 μM morphine [or β-endorphin-(1–31)] plus GTPγS ranging from 10 fM to 1 μM, significantly increased Gi2α-related immunoreactivity (fig. 4A). In the presence of GTPγS of less than 10 fM, the opioids did not alter the immunoprecipitates. Incubation of opioid agonists with 1 pM GTPγS (subeffective concentration), provided concentration-related increases of Gα-related immunosignals (figs. 4 and 5). Thus, GTPγS was required by opioid agonists to augment Gα-related immunosignals. The enhancement could be observed even in the presence of nonactivating concentrations of the nucleotide. This attribute preserved most of the trimeric G proteins for the study of agonist-mediated increases of Gα-like immunoreactivity. Therefore, the correlation of agonist concentration (gradual occupation of receptors) with the fractional activation of G transducer proteins was feasible.

Effect of opioids binding mureceptors in Gα-related immunoreactivity of mouse PAG membranes. A, The membranes were incubated with 1 μM morphine or β-endorphin-(1–31) in the presence or absence of increasing concentrations of GTPγS. * Shows a significant difference with respect to the corresponding Gi2α-like immunoreactivity obtained in the absence of GTPγS, P < .05, Student’st test. A wide range of morphine, β-endorphin-(1–31) and DAMGO concentrations were incubated with the membranes in the presence of 1 pM GTPγS. Solubilized samples (2 μg SDS μg−1 protein) were studied for immunoreactivity in agarose gels containing anti-Gi2α (B) or anti-Gx/zα serum (C). Immunoreactivity is expressed relative to the value obtained in the presence of 1 pM GTPγS and the absence of opioid agonists (percentage increment). D, Antagonism of naloxone on morphine-evoked increase of Gi2α-like immunoreactivity. The opioid antagonist was added 10 min before morphine to mouse PAG membranes in the presence of 1 pM GTPγS. Incubation was continued for 50 min at 25°C. Samples were processed for immunoelectrophoresis in agarose gels containing anti-Gi2α serum. Values are the mean ± S.E.M. of three to four independent determinations. Bottom, Morphine-evoked increase Gi2α-like immunosignals. Mouse PAG membranes were incubated with the opioid in the presence of 1 pM GTPγS. From left to right: 1, no opioid (basal); 2 to 9, 30 pM, 100 pM, 1 nM, 3 nM, 10 nM, 100 nM and 10 μM morphine.

Effect of opioids binding deltareceptors in Gα-related immunoreactivity of mouse PAG membranes. A wide range of DPDPE and [d-Ala2]deltorphin II concentrations were incubated with the membranes in presence of 1 pM GTPγS. The opioid DPDPE at 1 nM was added 10 min before [d-Ala2]deltorphin II. ICI-174864 antagonist at delta opioid receptors was added also 10 min before DPDPE and [d-Ala2]deltorphin II. Solubilized samples (2 μg SDS μg−1 protein) were studied for immunoreactivity in agarose gels containing anti-Gi2α (A and B) or anti-Gx/zα serum (C). Experimental details as in figure 4.

Whereas certain opioids displayed a weak activity, e.g.,DPDPE and [d-Ala2]deltorphin II on Gx/z proteins (fig. 5C), the basal immunoreactivity was almost doubled by morphine, β-endorphin-(1–31) and DAMGO (fig. 4C). By way of comparison, the apparent ED50s (mean ± S.E.M. from three independent determinations) needed for these opioids to produce one-half of their corresponding maximal effect were estimated (table 1).

The selective agonist of mu opioid receptors, DAMGO, activated the Gi2 and Gx/z classes of transducer proteins in a sequential fashion. [d-Ala2]deltorphin II and DPDPE selective agonists of delta opioid receptors activated Gi2better than they did Gx/z proteins. DPDPE was particularly weak in activating Gi2α subunits and inactive on the Gx/z type. However, this opioid appears to display greater efficacy in the activation of other classes of G proteins regulated by delta receptors (Laugwitz et al., 1993; Sánchez-Blázquez and Garzón, 1993).

In the presence of 1 pM GTPγS, 1 nM DPDPE significantly reduced (P < .05, Student’s t test) the ability of [d-Ala2]deltorphin II to increase Gi2α- and Gx/zα-like immunoreactivity, suggesting low efficacy of this opioid atdelta-receptors-Gi2 anddelta-receptors-Gx/z complexes (fig. 5, A and C). In mouse PAG membranes, morphine and β-endorphin-(1–31) exhibit comparable affinities for mu receptors of mouse brain (seeSánchez-Blázquez and Garzón, 1989). In this assay both opioids showed greater potency at Gx/z than at Gi2 proteins, indicating that they display dissimilar efficacy in the activation of G protein types.

Naloxone, devoid of an effect in this assay, reduced the potency of all the opioids. This antagonist incubated at different concentrations (0.1, 1 and 10 μM) impaired the ability of morphine to increase the immunoreactivity associated with both, Gi2α (fig. 4D) and Gx/zα subunits (not shown). The computed PA2and 95% confidence limits for naloxone were 7.9 (6.7–8.9), (KB = 12.6 nM) for Gi2α and 8.3 (6.4–9.9), (KB = 5.0 nM) for Gx/zα subunits. Values slightly higher than these expected for an effect just mediated by the μ-opioid receptors (Kosterlitz et al., 1980). Therefore, this result might indicate that in the PAG high concentrations of these opioids also act on delta-receptors (Sánchez-Blázquez and Garzón, 1989).

The selective antagonist of delta-opioid receptors ICI-174864 did not significantly alter the activity of morphine or DAMGO. However, it did diminish the potency of [d-Ala2]deltorphin II to dissociate Gi2α and Gx/zα subunits as well as the enhancing effect of DPDPE on Gi2α-related immunoreactivity (fig. 5, B and C).

Discussion

Our results provide a molecular basis for the explanation of the agonist-antagonist properties of opioids in promoting analgesia (Sánchez-Blázquez and Garzón, 1988; Garzónet al., 1994). It was observed that opioids displayed dissimilar “efficacy” at mu/delta opioid receptor-Gi2/Gx/z complexes because antagonist activities were revealed. Several agonists promoted a substantial activation of Gi2 and Gx/z transducer proteins at concentrations lower than those required to bind 50% of available receptors (KD) (Sánchez-Blázquez and Garzón, 1989). The levels of agonists required to activate a substantial quantity of G proteins in membranes from mouse PAG may be very low. In intact tissues these effects might be achieved even at much lower agonist concentrations. This could be explained by receptor reserve. However, the existence of receptors with a very high affinity for opioids, e.g., the μ1 (Pasternak et al., 1980; Lutz et al., 1984) must also be considered.

The involvement of opioid receptors in the observed opioid-evoked activation of G proteins was demonstrated. Naloxone, a competitive antagonist of opioid receptors reduced the enhancing activity of opioids on Gα-like immunoreactivity in a concentration-dependent fashion. ICI174864, antagonist of delta opioid receptors, displays negative intrinsic activity or inverse agonist activity on basal GTPase activity of, and binding of [35S]GTPγS to, Gα subunits (Costa and Herz, 1989; Georgoussi and Zioudrou, 1993;Mullaney et al., 1996). The concentration of GTPγS selected did not activate (bind) Gα subunits. Therefore, ICI174864 exhibited no reducing effect on basal Gα-related immunosignals. Interestingly, this antagonist exhibited high potency to reduce the activating effect of [d-Ala2]deltorphin II and DPDPE on Gα-like immunoreactivity. DPDPE showed even a greater potency to impair the effect of [d-Ala2]deltorphin II on Gx/zα-like immunosignals. The antagonism of DPDPE on [d-Ala2]deltorphin II-evoked effects has already been documented for analgesia (Vanderah et al., 1994). Thus, it seems that ICI174864 and DPDPE, after binding todelta opioid receptors, reduce the affinity of certain classes of G proteins toward GTP/GTPγS nucleotides. As a result of this inverse agonist activity, the efficacy of agonists in the activation of delta receptor-coupled G proteins is impaired. Further studies are in progress to explore this possibility.

The i.c.v. injection of pertussis toxin into mice reduces the efficacy of opioids in invoking supraspinal analgesia (Sánchez-Blázquez and Garzón, 1988). The antinociception elicited by the preferential ligands of muopioid receptors, DAMGO and FK 33824, of mu/delta receptors, [d-Ala2,Met5]enkephalinamide and [d-Ala2,d-Leu5]enkephalin, and of delta receptors, [d-Ala2]deltorphin II and DPDPE, appeared largely reduced in these mice. Most notably, the activities of morphine, β-casomorphin (1–4) amide and human β-endorphin were much more resistant to the bacterial toxin (Sánchez-Blázquez and Garzón, 1988, 1991). The results of our study and those of previous reports, confirm the involvement of pertussis toxin-resistant G proteins in the analgesic effects of certain opioids when acting at the supraspinal level (Sánchez-Blázquez et al., 1993; Garzónet al., 1994, 1995; Sánchez-Blázquez et al., 1995).

In pertussis toxin-treated mice, opioids for which activity was greatly reduced by this toxin behaved as antagonists of morphine-evoked analgesia (Sánchez-Blázquez and Garzón, 1988). Antagonism was also observed in mice that had received i.c.v. injection of IgGs to Gi2α or Gx/zα subunits (Garzón et al., 1994). It is well documented that morphine is less effective than other opioids that also bindmu receptors, e.g., DAMGO. This has been observed for the rat in the locus coeruleus (Christie et al., 1987) and vas deferens (Huidobro et al., 1980), and after chronic morphine treatment in the guinea pig ileum (Porreca and Burks, 1983). This evidence shows that opioids, after binding to their receptors, exhibit different efficacy in the activation of the different classes of G proteins. Variations in the relative abundance of these G proteins might be responsible for different pharmacological profiles and hence be interpreted in terms of diversity of receptors.

It is well established that pertussis toxin disrupts the interaction of receptors with G proteins as well as the direct activation by mastoparan of Gα-related GTPase activity (Weingarten et al., 1990). In membrane preparations containing ADP-ribosylated Gi2 proteins GTPγS failed to increase Gi2α-like immunoreactivity. However, even though GTPγS might bind pertussis toxin-acted Gα subunits (Winslow et al., 1986; Yi et al., 1991) our results, and those of other investigations (Sunyer et al., 1989), suggest that Gα subunits do not dissociate from the trimer. By fastening free Gα subunits to the trimeric G protein, pertussis toxin reduced the Gi2α-like basal immunoreactivity. Obviously this effect is not observed for the pertussis toxin-insensitive Gx/zα subunits. The stabilizing activity of GDP/GDPβS on the trimeric G-proteins also promoted reductions of Gi2α-related basal immunosignals.

In summary, the use of this sensitive, nonisotopic and quantifiable method allowed the detection of opioid-activated G proteins of mouse PAG membranes. Opioids showed different potency and efficacy in the activation of Gi2α and Gx/zα subunits. For opioids acting on the same receptor, the rank order of potency in activating Gα subunits depended on the class of G protein analyzed. The concept of intrinsic activity of ligands might therefore be expanded from the receptor molecule to the different receptor-G protein complexes.

Acknowledgments

The authors thank Drs. T. Fields and P. J. Casey (Durham, NC) for providing us with the recombinant Gzα subunit.

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